Vibrometry is the technical field of measuring vibrations of an object. In remote vibrometry, the vibrations are measured from a distance (aka no-contact vibrometry). One of the common ways to achieve vibrations remote-sensing is by using coherent electromagnetic waves (usually laser) and exploiting their physical properties.
Specifically, the vibrating object acts as a transducer by modifying the properties of the electromagnetic waves that hit it, according to the vibrations, prior to reflecting back the electromagnetic waves. As any sound source generates vibrations, coherent electromagnetic waves may be used to detect and sense sound. And indeed, many attempts have been made in the art of remote sound sensing and detection using coherent electromagnetic waves.
The majority of the coherent electromagnetic-waves-based sound vibrometers available today are configured so that the coherent electromagnetic waves are not directed at the vibrating sound source. Rather, the electromagnetic waves in these sound vibrometers are directed at objects that reflect the sound waves, usually flat surfaces such as windows and walls in the proximity of the sound generating object.
For example, U.S. Pat. No. 6,317,237 which is incorporated by reference herein in its entirety discloses a system wherein a laser beam is directed at a window pane of a building and the reflecting laser beam is received and analyzed to extract the sound waves (specifically human voices) generated within the building.
U.S. Pat. No. 5,175,713 which is incorporated by reference herein in its entirety, discloses a method for under-water sound sensing using laser beams directed at reflectors and analyzing the reflected beams in order to detect and sense under-water sound propagation.
Presently available remote sensing sound vibrometers use a variety of techniques to extract the information from the reflected beam. The traditional solution comprises an interferometer that conducts interference between the reflected beam and a reference beam. Another common technique is based upon the Doppler Effect. According to this technique, since the wavelength of the reflected beam is changed in accordance with the vibrations of the vibrating object that reflects the electromagnetic waves therefore the change in wavelength correlates to certain vibrations which in turn represent a specific sound signal.
Yet another technique involves the analysis of the speckle pattern. A speckle pattern is causes whenever a reflected beam of coherent light creates a spot containing a plurality of interferences. This results in a spot comprising varying intensity dotted pattern reflected from a vibrating surface. One of the ways to analyze a speckle pattern involves the use of a charge couple device (CCD) array or any other array of photosensitive cells serving as receiver units for the reflected speckle pattern, wherein digital signal processing methods help extract the sound signal.
FIG. 1 shows the general structure of a typical remote sound-sensing system according to the prior art. FIG. 1 shows a laser Doppler vibrometer 100 (LDV) which is one of the common embodiments for Doppler vibrometry. The LDV 100 transmits an outgoing laser beam 120 directed at a flat surface 140. The flat surface may be a window, a wall or a dedicated reflector that have been placed deliberately to act as sound reflector. A sound source 110 generates sound waves that hit the flat surface 140 which result in vibrations. The outgoing laser beam 120, upon hitting the flat surface 140 is reflected back to the LDV 100 wherein the properties of the reflected laser beam 130 has been modified due to the vibrations of the flat surface 140. Inside the LDV 100 the reflected beam is analyzed and compared with a reference beam (not shown) to reconstruct the sound that has been generated by the sound source.
The main drawback of currently available remote sound sensing systems is their poor ability of sound sources separation. This drawback is reflected in two manners: noise separation and blind sources separation. By relying on a beam reflected from a vibrating surface rather than directly the sound generating object, the systems according to the prior art are actually sensing the sound source's ambient, which may include noise that inherently reduces the quality of the sound sensing. In addition, by sensing a reflection from a surface, rather than the sound sources directly, the sound signal extracted actually represents the superposition of all the sound sources presented in the same close proximity. Noise filtering, as well as blind sources separation (the separation of the different unrelated sound sources) has to be performed using time-consuming and not always cost-effective digital signal processing (DSP) techniques.
It would be therefore advantageous to have an apparatus and a method that allows the physical separation of sources while monitoring the sound generated therefrom, as well as noise separation, without the use of complex DSP techniques, while retaining the high quality of remote sound sensing.